1. Field of the Invention
The present invention relates to a push-button switch and more particularly, to a push-button switch which is shifted from an initial or first OFF state to an ON state and then, to a second OFF state as the amount of depression of the push-button increases.
2. Description of the Background Art
In cases where, for example, a manual operation is performed on a numerically controlled machine such as a robot, an operator often enters a dangerous area to carry out his job. In such cases, a pendant with a push-button, such as called an enable switch (or deadman switch), is used for preventing the occurrence of an accident during the work.
This pendant is a portable unit which is enabled by connection with an operation device to teach a program to the robot or operate the robot. As shown in FIG. 80, the pendant 500 includes an input keyboard 501 disposed on a main surface and a push-button switch (enable switch) 502 disposed on one side surface thereof. Incidentally, the push-button switch 502 may be sometimes disposed on the rear side of the pendant 500. The pendant 500 further includes a signal cable 503 for connection with the operation device not shown.
As shown in FIG. 77, a conventional push-button switch 502 includes a push button 505 and a microswitch 506 disposed opposite to the push button. Disposed on a lower surface of the push button 505 is a leaf spring 507 extended downwardly therefrom. Disposed on a top surface of the microswitch 506 are a resilient push plate 508 and an actuator 509. A bent portion 507a is formed at a tip of the leaf spring 507.
When the push-button switch 502 is used, the pendant 500 incorporating the push-button switch 502 is first connected, via the signal cable 503, to a control panel of a machine to be manually operated. If the push-button switch is in the OFF state at this time, manipulating the keyboard 501 of the pendant 500 does not effect the key entry.
Upon subsequent depression of the push button 505, the bent portion 507a of the leaf spring 507 moving along with the push button 505 engages the push plate 508 of the microswitch 506, and the push plate 508 is resiliently deformed downward to press down the actuator 509, as shown in FIG. 78. This causes the actuator 509 to lower for establishing contact between contacts within the microswitch 506, thereby shifting the microswitch 506 to the ON state.
The operator keys in through the keyboard 501 of the pendant while keeping the push button 505 depressed for maintaining the microswitch in the ON state. If, at this time, the operator releases the push button 505, sensing the danger of contacting some moving part of the machine manually operated, the push button 505 returns to the state shown in FIG. 77 for turning OFF the microswitch 506. Thus, the machine is stopped.
In a case where the operator, who has panicked sensing imminent danger, further presses down the push button 505, the bent portion 507a of the leaf spring 507 slides on the push plate 508 to disengage therefrom, as shown in FIG. 79, so that the push plate 508 is returned to its original position by its restoring force. This shifts the microswitch 506 to the OFF state for stopping the machine.
Thus, the push-button switch 502 is adapted to enable the keyboard 501 of the pendant 500 or permits the key entry through the keyboard 502 for manual operation only when the microswitch 506 is in the ON state. Therefore, the operator's intent at the manual operation can be made distinct and hence, the operator's safety is ensured.
However, the known push-button switch is arranged such that the switch is maintained in the ON state by the engagement of the leaf spring and shifted to the OFF state by disengagement thereof which results from increased elastic deformation thereof. Accordingly, precisions of the leaf springs significantly affect a timing of shift between the ON and OFF states.
Therefore, the switch may sometimes be quick to be shifted from the ON state to the OFF state or slow to be shifted depending upon the variations of the leaf springs. Thus, the switch suffers from unstable operation and poor switching accuracy.